(navigation image)
Home American Libraries | Canadian Libraries | Universal Library | Community Texts | Project Gutenberg | Children's Library | Biodiversity Heritage Library | Additional Collections
Search: Advanced Search
Anonymous User (login or join us)
Upload
See other formats

Full text of "The cytogenetic relationships of four species of Crepis"

THE CYTOGENETIC RELATIONSHIPS 
OF FOUR SPECIES OF CREPIS 



BY 

JAMES A. JENKINS 



University of California Publications in Agricultural Sciences 

Editors : E. B. Babcock, W. P. Tufts, E. T. Bartholomew 

Volume 6, No. 13, pp. 369-400, plate 16, 3 figures in text 

Transmitted March 28, 1938 

Issued December 20, 1939 

Price, 35 cents 



University of California Press 
Berkeley, California 



Cambridge University Press 
London, England 



PRINTED IN THE UNITED STATES OF AMERICA 



THE CYTOGENETIC RELATIONSHIPS OP FOUR 
SPECIES OF CREPIS 



BY 

JAMES A. JENKINS 



INTRODUCTION 

The cytogenetical investigations designed to throw more light on rela- 
tionships and phylogeny of the various species in Crepis have progressed 
along two lines : first, an examination of the chromosomes of the various 
species ; and second, a study of hybrids. For the most part, the study of 
hybrids has been confined to those between more distantly related spe- 
cies, which, in the main, have been sterile. Consequently, the emphasis 
has been upon the cytology of the F x hybrids rather than upon the geneti- 
cal basis of the differences between the parental species. 

There are, however, a number of species groups the members of which 
are closely related morphologically and have a similar karyotype (Bab- 
cock and Cameron, 1934). From the morphological evidence, these spe- 
cies have had a common origin and apparently have not diverged very 
far from one another. The obvious conclusion is that the similar chro- 
mosome morphology indicates, in such closely related groups in Crepis, 
a fundamental similarity of the genes and their arrangement in the 
various chromosome types. 

The present paper deals with such a closely related group of species 
in Barkhausia, the most advanced subgenus of Crepis. Three of these 
species are insular endemics of Madeira and the Canary Islands; the 
fourth is a widespread species of northern Africa and Europe, which 
includes one endemic and one introduced subspecies in Madeira. 

The three endemic species are Crepis divaricata Lowe, C. Noronhaea 
Babe., 1 and C. canariensis (Sch. Bip.) Babe. 2 The fourth species is C. 
vesicaria L., and the two subspecies dealt with in this investigation are 
C. vesicaria taraxacifolia (Thuill.) Thell. and C. vesicaria andryaloides 
(Lowe) Babe. 3 

1 Crepis Noronhaea nom. nov. = Borkhausia (sic) divaricata var. pumila Lowe, 
Trans. Camb. Phil. Soc, 4:26, 1831; non C. pumila Rydb., Mem. N. Y. Bot. Gard., 
1:426, 1900. Named for Sr. A. C. de Noronha, Director, Museu Eegional, Funchal, 
Madeira, who sent the seed, collected in Porto Santo, from which experimental cul- 
tures were grown. 

2 Crepis canariensis (Sch. Bip.) comb. nov. = C. Lowei var. canariensis Sch. Bip. 
ex Webb et Berth., Phyt. Canar., 3:461, 1836-1850; BarTchausia hieracioides Lowe 
ex Webb et Berth., I.e. det. apud Lowe in litt., sed cf. Lowe, Fl. Mad., 1:559, 1868. 

3 Crepis vesicaria subsp. andryaloides (Lowe) comb. nov. = C. andryaloides Lowe, 
Trans. Camb. Phil. Soc, 4:25, 1831; Borkhausia (sic) hieracioides Lowe, op. cit., 
p. 27, no. 44 ; B. dubia Lowe, I.e., no. 45 ; B. comata Lowe, I.e., nt>. 46 ; C. comata Banks 
et Sol. ex Lowe, I.e.; Barkhausia hieracioides et dubia (Lowe) DC, Prod., 7:157, 
1838; C. hieracioides et dubia F. Schultz, Flora, 23:718, 1840; C. auriculata Sol. ex 
Lowe, Man. Fl. Mad., 1:556, 1868. 

[369] 




subspec 

A 


1 

SO 


OQ 


1 ' 


l 


bo 


Is sho 


1 - 


| 


1 






2 

O 
P< 

tub 



1939] Jenkins: Cytogenetic Relationships of Four Species of Crepis 371 

Crepis divaricata is found only on the eastern promontory of Madeira 
and there it is nearly extinct, owing to overgrazing by goats. C. Noron- 
haea is known only from Porto Santo Island, which lies to the east of 
Madeira. Its chromosomes were reported on by Babcock and Cameron 
(1934) under the name C. pumila, but this name is invalid. C. canariensis 
occurs on the two easternmost of the Canary Islands, Lanzarote, where 
it is abundant, and Fuerteventura. 

Crepis vesicaria andryaloides is also endemic in Madeira, being found 
only in the mountains along the north coast and occasionally on the steep 
slopes exposed to the sea down which it is carried by wind or water. It 
was finally recognized by Lowe (cf . Man. Fl. Mad. under C. hieracioides 
and C. andryaloides) as a highly variable species with many intergrad- 
ing forms, some of which were so extreme that he had previously given 
them specific or varietal names. For sake of brevity it will be referred to 
in this paper as andryaloides. 

Crepis vesicaria taraxacifolia is distributed in northwestern Africa 
and western Europe. It is polymorphic and several of its forms have been 
given specific names. A form which occurs in Portugal was described as 
a species (C. intybacea) by Brotero in 1816 and this form seems to have 
been introduced by the Portuguese into Madeira at an early date. There 
it was found and described by Lowe in his Manual (1868) as C. laciniata 
with two varieties, pinnatifida and integrifolia (the latter occurring here 
and there with the former, but less commonly) . Taraxacifolia is abun- 
dant around Funchal, the only port on the island, and in the vineyards 
around Boa Ventura on the north coast. Since it was found by Babcock 
along the trail above Boa Ventura, it is inferred that it has spread from 
Funchal to the north coast by this route. But it was not seen at all in the 
central highlands, so it probably has been carried by man or animals. 
The important point is that, having arrived on the north coast, it is 
hybridizing freely with andryaloides where the two come in contact; 
and it now seems probable that some of Lowe's perplexing forms (dubia, 
comata, and even the type of andryaloides) were the products of earlier 
hybridization. 

The main object of the present investigation was to study the five 
species or subspecies from as many different points of view as possible 
and particularly to state their relationships in cytogenetic terms. 

Acknowledgments 

This study was begun in 1931 at the suggestion of Professor E. B. Bab- 
cock, who supplied the material and facilities for the work. The writer 
wishes to thank Professor Babcock for his interest and guidance through- 
out the course of the work, and also to thank Professor R. E. Clausen and 
Dr. G. L. Stebbins, Jr., for their many helpful suggestions. 



372 University of California Publications in Agricultural Sciences [Vol. 6 

Acknowledgment is made of partial support of these investigations by- 
grants from the Carnegie Institution of Washington and the Rockefeller 
Foundation ; also to the Works Progress Administration for the services 
of a typist. 

MATERIALS AND METHODS 

The cultures used in the investigation were : 

(1) One collection of C. divaricata from the eastern promontory of 
Madeira, Promontory of San Lorenzo, Ilha de Cevada. 

(2) One collection of C. Noronhaea from seed collected in Porto Santo 
and grown for one generation in the museum garden at Funchal. 

(3) One collection of C. ccmariensis from Lanzarote Island in the 
Canary group. 

(4) One collection of andryaloides from the mouth of the Ribeira do 
Inferno on the north coast of Madeira ; the plants or the seeds had ap- 
parently been washed or blown down from the highlands. 

(5) Three cultures of taraxacifolia collected in the vicinity of Funchal, 
on the south side of Madeira. 

Collections of (1), (4), and (5) were made by Babcock in 1930, and 
the other two were sent to him at Berkeley in 1931. 

Crosses were made between all five entities in May and June, 1933, and 
repeated in the following year ; there was no obvious difference between 
the results in the two years. The method used was a slight modification 
of that described by Collins (1922). 

All root tips were fixed in Miintzing's (1933) modification of Nava- 
shin's fixative, section at 9/x, and stained either with Haidenhain's iron- 
alum haematoxylin or Smith's (1934) modification of crystal violet. All 
meiotic figures were studies in acetocarmine, McClintock's (1929) tech- 
nique being used. Pollen grains were mounted on a slide in a drop of 
acid fuchsin dissolved in lactic phenol, a medium which obviated the 
necessity of sealing the mounts. All the pollen counts were made after 
the plants had been in bloom for about two weeks. 

MORPHOLOGY OF THE PARENT SPECIES 

It is not to be expected that seeds collected from a few plants in the wild 
would give plants showing the total variability of any one species. How- 
ever, a comparison with specimens collected in the field showed that the 
cultures were a representative sample of the total variability of the 
species. The measurements can be taken as a fair approximation of what 
is characteristic of these species, and are adequate to establish the rela- 
tive differences between the species. 

The species were quite variable in themselves, as would be expected 
in self -incompatible ("self -sterile") plants. Canariensis and divaricata 
were more uniform than the other two, and it is interesting to note that 



1939] Jenkins : Cytogenetic Relationships of Four Species of Crepis 373 

the first two species are the most restricted in their range, and may be 
considered as relic species. 

In classifying the plants, there never was the slightest doubt of the 
species or subspecies to which they belonged. Each fluctuated about a 
distinct center of variability, and although there was frequently some 
overlapping in particular characters, yet in the sum total of characters 
the five entities were quite sharply delimited. The general appearance 
of all five is illustrated in plate 16, figures 7 to 12. 

The differences between the species were expressed in all parts of the 
plants, principally in small differences of size and shape. Some of the 
more distinct differences are summarized in table 1, for purposes of 
illustration. In addition to quantitative character differences, there were 
a number of discontinuous variations peculiar to each of the species or 
subspecies, as, for example, a purple tip on the ligules of canariensis. 
All these latter characters were of no obvious adaptive significance ; that 
is, they could be classified as nonessential. It is clear that the species can 
only be distinguished by means of a combination of characters. It was 
not possible, on the basis of external morphology of the plants grown in 
cultures, to divide the species into groups of a higher category. 

HYBRIDIZATION 

Dobzhansky (1937, p. 231) has used the expression incongruity of the 
parental forms for "mechanisms [including geographic and ecological 
isolation] which prevent the production of hybrid zygotes, or engender 
such disturbances in the development that no hybrids reach the repro- 
ductive stage." Conversely, we may define the congruity (or genetic 
"compatibility") of two forms as their ability to hybridize and the ¥ t 
hybrids to form viable gametes (that is, gametes capable of producing 
vigorous zygotes). There is no simple way of measuring the congruity 
or expressing it by means of a single numerical value. Two forms may 
be so incongruous that they fail to produce any hybrid seed, owing to 
their incompatible reaction systems ; or, on the other hand, they may be 
fully congruous, as is frequently true of varietal hybrids. Between these 
two extremes the incongruity may show up at various stages : the F x 
zygotes may be so weak that they fail to germinate, or if they germinate 
they may die before maturity; the F x plants may be quite as vigorous as 
the parental forms but closely approach complete sterility, for example, 
Nicotiana sylvestris x N. tomentosa, Clausen (1928), Primula verticil- 
lata x floribunda, Newton and Pellew (1929) ; finally, there are all de- 
grees of fertility of the ¥ x hybrids, and all degrees of vigor of the F 2 
zygotes. 

Consequently, an estimation of the percentage of viable gametes, in 
practice, entails : (1) a knowledge of the percentage of seed-setting on 



374 



University of California Publications in Agricultural Sciences [Vol. 6 



O 





c 


i 




O 


o 

<1 




w f 










DC 






% 






fe 


H 










(t 


Ph 




fe 


h 




•« 


p 




u 


CO 


T-4 


EG 


<^ 


h-3 


BE 


9 


PQ 


u 


s 


< 




B 






d 




l 


e 

< 




£ 


s 






5 




u 


>* 




lj 


g 




t 


£ 




c 


«J 




M 






Ph 






5 






c 





2 



° § 

CD >> -w 



0) 



03 CD 

QQ C3 
P tf 



2 t 

>00(ON 



d 

irf f-4 8 

i— i o *** 
d E "5 -° 

W ^ -M --5 



CD *H 

a § 

o d 
%>> 
~ <a if 

© is c 

02 « CD *H 
~ U> <D 

<D 2 •» +3 

.S o 
&0 d 



■as 



a § I 



0) 03 


CN 00 


K< 


<N IO tJH CN 

T— 1 



T5 


c 


d 

05 




n 


Si 


01 

c3 


a 


-m 


T3 


02 


<D 






5 °° 
7 i 

W5 O 00 © 
"5 CS M (N 



CD 

-^ CI 

2 -8 



c3 



§ 2 



.2 s 

1 & 

>> aT 

!S o3 

«3 *» 

3 CD 

02 « 
P 



P 0Q 



O 

© *! 

« CO 

O tO O <M 
lO C© »0 CO 



a 
13 >> 

p 

3 o3 

ft-r-T 

g I -s 

bo ft-** 
P 





c 






a; 




R< 


& 




n 


0) 




a 






>i 


aT 




; — | 


tf 


o 
a 


flil 


-M 


3 
CO 


02 



CQ 


* fl 2 ». 




13.0 c: 
6.0-1 
3.2 
2.3-4 



03 CD 

•43 a 




fe 



to u 

^ 3 S 
03 02 n 

I -2 

>j 02 *rl 



CD 






?3s 

^ ft 

02 «2 
P 



"a § 






•a « 02 

02 02 
nfl cu 

5^ a 



o a 

a, cq 
02 
02 



O 



a a -s, a o 5 



P 



ft ^H 



J rd 



i" d 



T3 d rd "H o3 

03 02—1 rj . = O 

Sm d d d 

r#T CD O H . 

!» 4s be 02 _- >^ a 

CD S. 03 02 g fl § 

g .d •+= o -p «s -p 

0Q 



<u 



c3 

a.. 

d ^ 
d^ 

CT 03 
CD^ 

fa 



1939] 



Jenkins : Cytogenetic Relationships of Four Species of Crepis 



375 











+> 


>> $ 




j>> 




d 


— GO 








03 


ar or 
; rare 
ing ba 




ricate 
at the 
basal 




i— i o3 


03 03 G, 




•— ! 03 7-2 


««^T IO 


0. 03 


jS jS o3 




^4 


03 b 

i—i 03 


^ 03 _ 


"* 


"° pCI fe 03 


T-H ^ 


bC fn 


S o -3 


CN 


— ! fl 03 c3 


gH qj 


r— ] f_i 


03 fl ."2 
1-3 S 
P 


O CO T 

CO io o^ *o 

ooNowdd 

<N i-t CO rt 


Usual 
bra 
bas 
foli 


o3 b 

3 c3 

' DQ H 
P 


13 

02 rM 
P 


£ 2^ 




02 1 






lu S iH 




!>> £ > 




Sh 


sually cordate, 
quently oblanc 
late; usually w 
clasping base 






o3 


-77 

-50 

.5 

.9-3.6 


Usually regular] 
branched upw 
with foliage e 
ly up the stem 


CO f 

>>^ 
r-H Q 

o3 b 

3 03 
co *-* 


ways glandul 
hairs, usually 
blackish setae 


P 


N On* (N H o 


p 


< 


o o£ 




TH S3 






date, 
s lance 
blance 
lly wi 

•ase 




larly 
pwar 
e eve 
em 


CO 


3 




P 3 bfl -+? 


"•+3 c3 
03 S 


*"d 


ally cor 
metime 
te, or o 
te; usua 
asping b 


r- ^ "P 

O O 1 


ally reg 
anched 
th folia 
up the s 


1 

3) 

E? 02 
03 "3 


§ £ *:*3 
P 


00 «D (O N 
CO lO IO CO rH © 

^ M N h 


g.3 ** 
P 


d 02 

I S 


S. o3 


>» 




tBi 




. >> 


linear or 
late, rare 
eolate 




03 S 

-d 2 

11 


f: 


glandular 
frequentl 
sh setae 


>5 O fl 


t^ 


sually 
basew 
liage 


* j>? 


02 mM 


£3 C3 03 

3 g 3 
1.5 o 


»o CO 1 

CO -* O "5 


r-J 03 

03 b 

3 03 

02 H 


way 
hair; 
blac 


p 


CO 00 »0 CO rH © 

INHNH 


P 


P 


< 


1 

a, 

CO 

oa 




"§£ 








03 S 


*0 


o3 


3 




4 s 


ef 4 

rH 02 

03 

s a 

.§•■§ 

a ° 

g 02 


3 


£ 03 
03 § 

^ C 
O — ' 

o 


cm. 
-37 

cm. 
-40 
.0 
.8-1.3 


sually branc 
base with b 
liage 


1 

"So 

DD 02 

g 'S 


© 1> O IO rH o 

CO CM CO CN 


P 


o 


5 




•-^ «r> 




02 
03 


, 


i Leaves 

of the middle 

ine leaves 


e) 
) 

e) 

d (mdn. 
(range 




^FLORESCENCE 

Number of heads on 
the ultimate branch 


r3 « 
O 03 

11 


rEM 
Height (mdn 

(rang 
Spread (mdn, 

(rang 
Height/sprea 


b/0 

3 


O q) 
03 ft 

2 ^ 

ci a 

03 03 


AULINE 

Shape 
caul 


C3 

q 

o3 


03 03 
_D rH 


O 


m 




►H 





376 



University of California Publications in Agricultural Sciences [Vol. 6 















b i 
















^ o 
















d TJ 
















3 
















co d 




e 












d o 




1 












I.& 




o 












l-H ^ 




a 

H 


© 








o 


ff « rSS 




(3 


© 


o 


•* 


co 


'tf' 


CM 




i-H 


°? 




CO 


rH 


CM 




9®. 


CO CM 


CM 


00 © 


IO o 


a S * 

S 03 (B 


O CO 








*C CM 










Oi »C 


I— ( T— 1 


»0 CM 


CM CM 


CM d 


CD 
r-5 


CM r-i 








r ~' *~ ' 




1— 1 T— * 
















&T a 
















O » 
















a .13 -v 




2 












CD pd g 














>> ^ CD 




"3 

•1 


o 


© 




00 


CO 


111 


CM 


s 






CO 




CO 




e 


9* 


csi 


^ 


^ 


T-H 


^d r2 * 

6X),0 ^ 
r3 


CM 




9 9 


CO o 


© © 
i-h CM 


1 

CO CO 


9^ 


9 9 




00 "*' 


1— 1 1— I 


CM CM 


<■* © 


CM i-i 








Tt< i-H 




rH rH 
















CD CD 
















.2 CD 

S -5 d 

9 d h 

° 5 -a 




£ 














e 


o 








O 


r-l O « 






<m 


CM 

CO 4< 


1—1 
00 
1— ( 

CM CO 


00 


© © 


3 ® 


O 

i 

*o CO 




r-i t> 


I— I T— I 


CO r-l 


CO <M 


od d 


S " 


<M CM 




i—i 




i-H i-H 




rH i— 1 
















1 00 •* 


CD 














© CD CD 


.Oh 














►» a a 

° CD 2 

11" 

co d 

>> o 


H-> CD 

^O .i-i 
OJ CQ 

^ — i 


£ 












o3 03 




o 


00 




co 


© 

d 


a c3 9 


r— 1 


1—1 


00 


CO 


1—1 


§ O CD 


03 -rj <M 




oi 


CO <* 


T— 1 

1 


© »C 


9o 


S3 d cm oo 








00 © 






do • 




© t^ 


rH i—l 


•*f o 


CM r-i 


Tji rH 


P "" 


CM rH 








1— 1 T— 1 




i— 1 rH 
















,d 
















+a 
































t* 
















-j 




05 












CD 




00 












a a 
















S'-g 




c 

(3 

e 
e 
6 


a 9 
o2 


CO 

1-2 


CM 

CO 


o CO 


a° 
a 7 
9 9 


^3 ® 

ft 

r-l 


a> 
a <^ 

CO CM 








i-i Oi 










00 *o 


i—i i— i 


TJH O 


CM <M 


CM CM 


CM CM 








1— 1 1—1 




i— i i-i 








^-s <■ — \ 
















• CD 














«J » 


d &o 


bfi 












1 -° 


32 fl 


a 












si --? 


a g 


*-j3 
9 v„ 


O CD 


O'o - 




^"^ a) 




^~^ 3 . CD 
8-75 S S g 


rd 


oi bfi 


a bo 


d WJ 




B d 8 


h 

i 




*7! c 

d CD 

o * 


73 d 

a | 


a i 

\ — * **s 




o -3 1 

1 S^ 

CD »^^ "^^ 

d 


o 


CO ^ °3 bfl 

1 i s a 

r- 1 


rd 

bfi 

a 
pq 


02 "+■' 

>> o 

03 ■*=• 
Q 


CD 

Si 


rd 
3 1 

9 CD 

rJ 


rn 
O 

o 


o3 ^d. 

-9 to 

i § 

02 



1939] 



Jenkins: Cytogenetic Relationships of Four Species of Crepis 



377 



a 

5 










d 
5 








cu 










o 








H 










t-i 








? 










X2 








cu 


»o 


»o 






d 
o 


o 


o 


o 










O 


a 

o3 


d 






o 
cu 


CO 
O <N 


^ CO 


op 


IO © 


00 4< 


id 


i 

OS IO 


"3 

Oh 


eo csi 


d d 


t» CO 


© © 


b 


t^ d 


CO <N 


rji CO 












02 


















^ d 








d 










§^S 








cu 

CU 










« -a 2 








H 










d ,o 








f 










>> 3 g 








CU 










s --* 








> 


o 


IO 




o 


d d >> 


IO 


o 


t>. 


Oh 


IO o 
eo eo 


d d 


00 

co 4< 


oo od 


1 1 1 


»c o 
io id 


J 

00 o 

t- ( rH 


O IO 

rjl CO 


-P 


















■ 










d 








'S 










E 








CU 










o 








CU 










h 








H 

b£ 










^2 








£ 


o 


oo 






CU 


t^. 


IO 


CM 


+3 


>d 


d 






>> 


00 


« 


up 


,4 

bO 


00 © 


b- CO 


i— 1 


o o 


73 

d 


IO CO 


CO o 


IO IO 


53 


Tti <<*' 


d d 


1 
OS t> 


CM CM 

l— 1 i-H 


03 
> 


d id 


r-t i— 1 


"*' CO 












<U 02 


















^ cu 


















>> a 


















d -? 


















a s d 


















> a * 










o 


o 




o 


ill 


o 


o 


IO 


o 


i 

CO <N 

CO CO 


CO «o 
d d 


«D iO 


CM 

H © 

T-H 


I 8 J 


5 

oo © 
•d <* 


IO o 
I— 1 1—1 


Tji co 












d 


















J 2* 










a *> 


S *: 




a 




a 9 


a n 


a 9 




(M O 


CO IO 




a 

o o 


a «p 


a <n 

"^ Oi 


a «p 

CO i>. 


CU 


CO CO 


d d 


o o 

1— 1 T-H 


d d 


02 ■"" l 1 


IO Tt< 


r-i d 


CO CM 








Ft 

CD 

§ ^ 1 
» a s 

-*=> w w 








O'aT 


O'cu 








-d 






d bfi 


d bfl 






OQ 


+3 






T3 d . 


^ d 




/ ~ s cu 


CU /-N 

be ^* cu 


bfi >~v / ~" N 




O CU 


a 2 


a 2 




C W) 


o3 o bC 


S d bo 




d be 


"W ' V ' 


s * s_^ 






"SI 1 


a, G Sh 




"9 S 

a 2 


rd 


bfi 




-3 


%■$ 


o3 


o3 






bfl 

d 


cu 


u 

o 

"3 


1 -*» 

s- bfi 


1 -4-3 


« ^ 


-O 


02 


J=l 


cu 


02 


2 fl 
— cu 


2 c 

^3 CU 


p o3 


cu 


i © 

go 


bC 


^ 


3 

a 


O 


^ . 


^ 


O 3 


1 

1— 1 


cu 


o3 
CU 


o3 




"< 


<< 


h- 1 


^ 


pq 


Ph 



378 



University of California Publications in Agricultural Sciences [Vol. 6 



the F ± plants, and (2) the raising of an F 2 population in order to de- 
termine whether or not the gametes are capable of producing vigorous 
zygotes. For a complete conception of the congruity there are also neces- 
sary: (3) an estimate of the percentage of hybrid seed obtained from 
crossing the species, (4) a record of the germination of that seed, and 
(5) a knowledge of the percentage of ¥ t plants that grew to maturity. 
The data on congruity for the ten possible hybrids are given in table 2. 
It might be well to discuss at some length each of the measures. 

TABLE 2 
Percentage of Hybrid Seed-setting, Percentage of Germination of the Re- 
sulting Seeds, Percentage of Morphologically Good Pollen on the Fi 
Plants, Percentage of Open Fertility of the Fi Plants, and Germination 
of the F 2 Seeds, of the Ten Possible Hybrids 



Cross 


Hybrid 
seed-setting 


Germination of 
hybrid seed 


"Good" pollen 
on Fi 


Fi Fertility 


Germination 
of F2 seed 




1 


* 


5 


4 


5 


T-A* 

N-A 


54 (l)f 
44(2) 
42(3) 
42 (4) 
38 (5) 
36 (6) 
36 (7) 
24(8) 
16(9) 
10 (10) 


71(5) 
86(1) 
76 (3) 
62(7) 
43 (10) 
68 (6) 
48(9) 
75(4) 
53 (8) 
86 (2) 


81(1) 
46 (5) 
44(6) 
10 (10) 
19(9) 
58 (2) 
56 (3) 
33 (8) 
35(7) 
53(4) 


FairJ (3) 
Fair (4) 
Poor (8) 
Good (1) 
Very poor (10) 
Good (2) 
Fair (5) 
Fair (6) 
Poor (9) 
Fair (7) 


35 

88 


N-T 


20 


D-A 


47 


D-N 


54 


D-T 


67 


D-C 


43 


A-C 

N-C 

T-C 


50 








Average 




34 


67 


44 


Fair 


44 



* T, vesicaria subsp. taraxacifolia; A, vesicaria subsp. andryaloides; N, Noronhaea; D, divaricata; C, 
canariensis. 

T-A includes both combinations, namely, taraxacifolia 9 X andryaloides c? and andryaloides 9 X 
taraxacifolia d". The other nine combinations also include the reciprocals. 

t The numbers in parentheses refer to the relative order of the observation in magnitude array, begin- 
ning with the highest. 

t Excellent, 76-100 per cent; good, 51-75; fair, 26-50; poor, 3-25; very poor, 1-2. 

The percentage of hybrid seed-setting (table 2, column 1) is merely 
the ratio, expressed in percentage, of seeds obtained to the number of 
florets emasculated. Approximately one hundred florets were emascu- 
lated in each cross, with conditions kept as nearly constant as possible. 
No difference was noted in reciprocal crosses. It may be significant that 
when the percentages are arranged in magnitude array, crosses between 
taraxacifolia and andryaloides stand at the top ; these two subspecies are 
hybridizing in nature and are producing many intermediate progeny. 
There is practically no difference between the various divaricata and 
Noronhaea crosses, and finally, the crosses involving canariensis are all 



1939] Jenkins : Cytogenetic Relationships of Four Species of Crepis 379 

at the bottom of the list. The numbers and the samplings of the native 
populations of the species are not sufficient to establish any precise de- 
gree of relationship on this basis. 

The percentage of germination (table 2, cols. 2 and 5) of the F x and F 2 
seed was very little different from that of the parents ; the average germi- 
nation for the four species, including the two subspecies of vesicaria, 
was 45 per cent — varying from 25 per cent for canariensis up to 86 per 
cent for andryaloides. There was no obvious difference in the reciprocal 
crosses, and the average for the various crosses did not differ markedly 
from the average for all the crosses. Practically all the ¥ 1 seeds that ger- 
minated grew into plants which lived to maturity ; the few that died in 
the course of the experiment did so from causes far removed from in- 
harmonious combinations of genes. 

Fertility (table 2, col. 4). — A study of the fertility of the hybrids is 
complicated by the situation in the pure species, where, with the excep- 
tion of subsp. taraxacifolia, the parents are completely or almost com- 
pletely self -incompatible. However, an abundance of seed was obtained 
when the heads of sister plants were rubbed; also, the fertility of the 
open-pollinated plants was high, particularly when exposed to the visi- 
tations of insects. 

An attempt was made with canariensis to see whether there were 
definite intrasterile-interfertile groups within the species, as East 
(1932) and others obtained in Nicotiana and other genera. The results 
did not conform to a simple scheme. If there was a single series of fer- 
tility allelomorphs, their clear-cut expression was modified by other 
factors, either modifying genes or fluctuations of the environment. 

It was difficult to measure the degree of fertility accurately ; so the 
percentage of seed-setting was estimated as belonging to four groups : 
exceUent (76-100), good (51-75), fair (26-50), and poor (1-25) ; the 
last class was subdivided into a classification of very poor (1-2) . 

The open seed-setting on the F x hybrids was markedly poorer than 
that of the parents, where the open seed-setting was usually 100 per cent, 
or at least in the excellent class. Also there was a decrease in the average 
amount of morphologically good pollen (table 2, col. 3) ; although, even 
in the parents, which usually had from 80 to 100 per cent of apparently 
good pollen, sometimes there was as low as 20 per cent, in spite of the 
fact that the flowers were selected when the plants were at the height of 
their blooming season (that is, after they had been in flower for about 
two weeks). As a consequence of this variability, the amount of good 
pollen could not be used as an index of fertility. If there was some simple 
relationship existing between the amount of apparently good pollen and 
the seed-setting, it would have been very difficult to establish without an 
extensive statistical study. 



380 University of California Publications in Agricultural Sciences [Vol. 6 

Under the most favorable conditions of seed-setting, open-pollinated 
heads sometimes appeared on some F 1 plants in which all the possible 
embryo sacs had developed. Consequently, the normal procedure in 
gonogenesis being assumed, there is every indication that under certain 
circumstances all the female gametes in these particular hybrid plants 
are capable of functioning. 

Conclusions from hybridization. — (1) All five entities hybridize very 
readily. (2) The crossability of the various species and subspecies, taken 
in pairs, was about the same, though there was a suggestion that cana- 
riensis was less congruous ("compatible") than the others. (3) The 
hybrid seeds germinated as well as those of the parents, with little or no 
difference between the individual crosses. (4) The hybrids,- on the whole, 
were less fertile than the parents, and there was less morphologically 
good pollen, but the correlation was not obvious. (5) Hybrids involving 
taraxaci folia were slightly more fertile than the others. (6) Under cer- 
tain circumstances, all the gametes in some hybrids were capable of 
functioning. 

CYTOLOGY 

All five species and subspecies had eight chromosomes at the mitotic 
metaphase, confirming the observations of Babcock and Cameron (1934) . 
Although several plates from each were carefully measured, there 



-& 



it/ 

w a b 

Fig. 2. Somatic metaphase from root-tip cells of : a, Crepis vesicaria 
subsp. taraxacifolia; b, Crepis canariensis x C. vesicaria subsp. andry- 
aloides F,. x 2500. 

All parts of this figure were drawn with the aid of a camera lucida at a 
magnification of 3750 times, from permanent preparations, and reduced 
one-third in reproduction. 

seemed to be no constant difference either in morphology or in total 
length of the various chromosome types. The differences observed be- 
tween them were small and could easily be explained on the basis of 
variations in fixation, age of the cells, twists, etc. The somatic chromo- 
somes of taraxacifolia are illustrated in text figure 2, a, which would 
serve equally well for any of the parents (see also Babcock and Cam- 
eron, 1934). 



1939] Jenkins : Cytogenetic Relationships of Four Species of Crepis 381 

In the first-generation hybrids the somatic metaphase chromosomes 
appeared to be the same as in the parents. In size, staining capacity, and 
morphology the maternal and paternal elements could not be distin- 
guished. Both of the D chromosomes in the hybrids had a satellite, that 
is, there was no indication of amphiplasty as reported by Navashin 
(1928) and Hollingshead (1930) in more distant species hybrids in 
Crepis. Text figure 2, b shows a somatic plate of the F x hybrid canaden- 
sis x andryaloides, which is essentially similar to the parents. 

Meiotic chromosomes. — Both in the hybrids and in the parents, at first 
meiotic metaphase four bivalent chromosomes were regularly seen, all 
of which disjoined in a normal fashion (see text fig. 3, b and c). Also, 
the second meiotic division was normal, and comparable, in all respects, 
in the parents and in the hybrids. 




Fig. 3. Meiosis in Crepis Nororihaea x C. canariensis Y t : a, diplotene showing 
four paired elements; b, metaphase, showing four typical bivalent chromo- 
somes ; c, anaphase, showing four chromosomes passing to each pole, x 1700. 

All parts of this figure were drawn with the aid of a camera lucida at a mag- 
nification of 3400 times, from acetocarmine preparations which were squashed 
by light pressure, and reduced one-half in reproduction. 

The parents frequently have a single nonterminal chiasma at early 
diakinesis (see text fig. 3, a) , the minimum required to maintain pairing. 
Earlier stages were not examined in detail in regard to chiasma fre- 
quency, as it is almost impossible to distinguish between twists and 
chiasmata in acetocarmine preparations. At late diakinesis there was 
usually one, sometimes two chiasmata, and rarely three. This is rather 
surprising in view of the great length of the Crepis chromosomes. An- 
other curious fact is that there was little terminalization until late 
diakinesis or early metaphase. 

Recently, Darlington (1931) has regarded the relative frequency of 
chiasmata formed in the first meiotic division of the parents and the 
hybrids between them as a measure of the genetic homology of the chro- 
mosomes. The work of McClintock (1933) on nonhomologous association 
and Beadle (1933) on asynaptic maize, and of Kihara (1929) and others 



382 University of California Publications in Agricultural Sciences [Vol. 6 

on the influence of temperature, would throw considerable doubt on this 
measure of relationship. Nevertheless, if this criterion has any value 
whatever, these species are very closely related. 

The cytological evidence strongly indicates that the five entities have 
a similar arrangement of genes in the various chromosome types. In 
other words, there have been no large duplications, translocations, or 
other rearrangements that in any way interfere with normal meiosis. 

HYBRID SEGREGATION 

The five entities were crossed in all possible ways, making ten different 
hybrid combinations, each including the reciprocal hybrid. F 2 cultures 
were grown from all except two combinations, namely, taraxacifolia- 
canariensis and Noronhaea-canariensis, in which no self ed or sibbed seed 
was obtained. The behavior in all the hybrids was remarkably similar 
and of a type frequent in crosses between closely related forms. For the 
sake of brevity, the general behavior of the hybrids will be described and 
illustrated by data from only one hybrid combination, namely, taraxaci- 
folia-divaricata. 

The Fi-generation plants were variable, but no more so than the par- 
ents. The character differences were manifested in either of two ways : (1) 
they were more or less intermediate between the parental averages, or 
(2) the influence of one parent was more pronounced. The greater num- 
ber of the characters were of the intermediate type, which included prac- 
tically all the distinctive species differences ; for example, height, flower 
size, achene size and color, and beak length. Each hybrid was distinctive, 
and it was easy to determine the parental species by an inspection of the 
hybrid; thus no species was entirely dominant over any other species. 
The characters that showed dominance were, for the most part, those 
which had no apparent adaptive significance ; for example, anthocyanin 
patterns, pubescence, color patterns (see pi. 16, figs. 1-6 for the appear- 
ance of the rosettes of another series of hybrids) . 

The Fi-generation plants were just as vigorous as the parents, but no 
more so. This lack of hybrid vigor is probably to be explained by the 
consistent cross-pollination of the wild species, which makes them highly 
heterozygous. 

In the F 2 , most of the characters followed the blending type of inheri- 
tance, even most of those in category 2 above. This shows that the specific 
and subspecific complexes were made up of a large number of genes, and 
that most of the characters, if not all of them, were influenced directly 
by many genes. A few characters, those determined by a single gene 
differential, showed dominance in the F 2 . 

The characteristics of the blending inheritance in these species hy- 
brids, as illustrated in tables 3-9, may be roughly summarized as follows : 



1939] Jenkins : Cytogenetic Relationships of Four Species of Crepis 383 

(1) In both F x and F 2 there was a continuous range of expression of 
any one character difference, with the majority of the individuals inter- 
mediate. The mean of the F 2 population was similar to that of the F 1# 

(2) The range of the F 2 variation was about that of the parental ex- 
tremes, with no well-marked occurrences of transgressive inheritance. 
This lack may have been due to the small numbers, as the F 2 population 
in any one cross did not exceed one hundred individuals ; on the other 
hand, it may have been due to the lack of dominance in the various gene 
series. (3) There was no recovery of types corresponding to the parents 
in all or most of the characters. In many hybrids, particularly where 
there are chromosomal difficulties, the intermediate types are eliminated, 
that is, they are unfavorable combinations. The fact that no parental 
types were recovered indicates (a) that the intermediate combinations, 
or at least a large number of them, were able to survive, and (b) that 
there were a great many genie differences between the two parents. 
(4) No new characters appeared in either F x or F 2 , which would indicate 
that the gene systems in all the species and subspecies were essentially 
the same. The new combinations in the hybrids merely altered the ex- 
pression of the existing characters. 

It is now known that multiple genes can bring about such a continu- 
ously varying F 2 , that is, several genes, each with a small increment, 
affecting the same character. It is very probable that species that are 
highly heterozygous will have a blending type of inheritance, or at least 
a variable expression, for most if not all characters. It is only in rela- 
tively homozygous lines that it would be possible to obtain a sufficient 
number of clear-cut segregations to reveal the genetic basis of such small 
character differences as those found in these species. It would be a long 
and tedious piece of work to put these interspecific differences on a 
Mendelian basis, and the task would be greatly complicated by the pres- 
ence of self -incompatibility. 

Data were taken on a number of more or less clear-cut character differ- 
ences between the species and subspecies. With the small number of in- 
dividuals, and in the limited time, it was not possible to work out the 
factorial bases of these characters beyond saying that they are gene 
determined but do not conform to a simple Mendelian scheme. The re- 
sults in one are more or less typical of them all, and for the sake of 
brevity only one will be illustrated. 

Both taraxacifolia and Noronhaea have a conspicuous red stripe on 
the dorsal surface of the outer row of ligules, which was somewhat vari- 
able in its expression. The other three, namely, canariensis, divaricata, 
and andryaloides, had no stripe, and it did not appear in any of the six 
possible hybrids between them. 

In all the crosses in which one parent had a stripe and the other had 



384 



University of California Publications in Agricultural Sciences [Vol. 6 



< 

CO 

a 

g 

a 

ft 
Q 

H 

K 

<5 
ft 

pi 

02 

m 
p 

a a 

a H 

*! 

gn 

s « 

g P 
a B 

B I 
ft 9 

H S 

>-h 



o 

a 
P 

w 
3 















5 

£ 


o O CM i>- 

^ CO CO CM 

1— t 


CD 

cm 


T-H 




TH tH CM 




CM 


CO 


(M CM 


CN 


H CO <M 


eo 


H -* OO 


o 


TJH Tj< CO 


OJ 


N iO l> 


00 


CO 00 rH IC 




T-H 


t~ 


>OHNN 




r— 1 


to 


CO *0 CM "ti 




1— 1 


«o 


CO N CO CO 




i— i 


T»* 


CM t>- rjH CM 




r— I 


eo 


CO N CO 00 


<M 


co co ■* >o 


,M 


lOHiOO 




T— 1 


o 


CO «3 t* 


* 


i—i eo 


02 












X) 




































X 












>> 












£ 












13 












c3 




o3 






03 
CD 


o3 *o 








-M uZ 








o3 '13 






a 


8 2 






CO 


.5 c3 
3 1 








•5i 3 - . 




P 


h 


fe 


U- 





o 

Q 

< a 

Eh g 

** a 






CO 



£ a 

< PQ 

<-} w 

ft o 





H ft 




N S 


"tf 


O H 

§ s 

is 8 




H 


S w 




ft 8 








a ft 




ft co 




o a 




Bfi 




■-i g 




a ft 




ft Q 




fe g 




o < 




o ~ 




P < 




ft M 




*< d 




K 1 




1 1 

ft H 




fe «e< 




O « 




CO g 

S5 ft 



u 

|Z| 

B 
g 

a 



^ 










O O CM l> 


o 


rt* CO CO IN 


ft 


*-* 






<r> 


T-H 














o 


CM 


00 
"5 


CM »0 


CD 


H t-H CM 


U3 


CM CO 




*C -«*< O 


O 


»o CO 

l-H 


00 


t^ CM Ci 


CO 


i-H OS CM CO 




CO OS ^ -^ 

l-H 


64 


CM CO -* CM 


o 


OS CO *0> OS 


00 

eo 


■* ti iO CN 

T-H 


CO 

CO 


*0 CM i-i OS 


eo 


io eo >o 


eo 


CO CO t-H »C 


O 
eo 


<* "* 






es 


^ 


* 




CO 


T-H 


eM 




E» 




T3 
























X! 


: 03 






>> 








A 


o3 *o 






T3 








fi 
o3 


03 -g 
.2 o3 






s 


H X 






o 


o3 c3 






> £3 






0. 


•H 03 H < 


OQ 


QHPh 


£- 





1939] 



JenJcins : Cytogenetic Relationships of Four Species of Crepis 



385 



<; a 

8 ■ 

03 H 

t 
3 pq 

« OD 

9 s 

g « 

g w 

a a 

M B 

1 H 
Q JL 

I 8 

a w 















e8 


CO <N O CO 


O 


(N MM O) 


00 


«H 


TJ< 




«© 


T-H 


"* 




TJ» 


CO 


■* 




co 


Tt< 


"* 




o 


<N CM <N 


■^ 




00 


(M <N 


CO 




<o 


CM M H 


CO 




<« 


Tt< *0 *>• 


eo 




CM 


<M <M 00 CM 


CO 


CI 


© 


CM O CD "^ 


CO 


T-t <N 


00 


N ^ CO N 


e* 




«© 


»C !>. 


e>» 




■* 


h if) h rl< 


e» 




co 


M (N N 


CO 




o 


CO T* 


CM 




• 

oo 


^ 


1-1 




so 












T3 




































X! 












>> 












J2 










TJ 




53 






C 


J2 






s 

I 


8"S 

03 o3 

> *-< 












P 


E- 


fn 


pt 





& 
OQ 

g a 

2 B 

§ ft 

3 w 

8 I 

> B 

d« 

« 8 

3 ^ 



O 



^ 



_ 












o 


h S O M 
(M CM CO O 


H 


T-H 






00 




«o 


CO 






o 


^ 






US 


CO 






© 


i-i o 


o 




lO 


ta io 






§ 


N NO00 


•*< 


00 CM -^ OS 


o 


N W D »C 




T-H 


•O 


CO CM »C i-l 




1—1 


© 


i-i CM OS 


CO 




«o 

CO 


OS CO CO 


© 


"* oo 






• 


T-H T— 1 


















T3 
























c 












£> 












>> 












A 












T3 












a 












cj 












s 
























0) 




o3 






a 










GG 


o3 IS 

jl 

c3 c3 
> *3 








■Si o3 - . 


1 


P 


r- 


I* 


fe 





386 



University of California Publications in Agricultural Sciences [Vol. 6 





H 




H 




H 




Q 




fe 




<J 








<5 








s 




p 




s 








o 




s 




«d 




tf 




< 




Eh 




A 




02 




B 




02 




"«j 








s 




<1 




O 




02 S 




£g 




r-; H 




° Z 




Q H 




£ P 




< f 




< i 




y « 




g « 




« s 




5 3 


t^ 


!w 


H 


<-> * 


pq 


Sg 


< 


9 Si 


H 


a w 
h g 




s & 




Ph h 




02 O 




o o 




H £ 








2 ™ 




3 p 




W § 








O P5 








^N 



tf 



1 


o 

CM 


£ 


o 

CO 


§ 




H 


1— 1 


«* 


1— 1 


CO 




e* 




eo 




o 


1— 1 1—1 


CO 




00 


CM 


<N 




to 


CM H T)< 


<N 




*<t< 


NNHM 


<M 




CI 


i-i *>■ 


<M 


CM 


O 


H MM lO 


e* 




oo 


CM <M eo 


" 




to 


H M N >0 


1-1 




•«*< 


H rt MOO 


~ 




<M 


i-i 00 »o 


" 




O 


CO (N M rt< 


** 


i—i 


00 


M H •* Tjl 


o 


1—1 


to 


CqNMO 


o 


1— ( 


• 


co os 


© 


1—1 


m 












T3 




































,Q 












>> 
















ca 






d 








3 


03 o 






.2 
'o 

s 


!l 






§ S 








> fH 








•h d - , 




c 


E- 


fr 


fc 





g * 



o 



<1 GO 
ft 

o 

w 

G 

S3 
H 

ft 

O 

03 

o 



fe 



3 


<M O 


oa 


r>. 




£ 


MMHO) 


o 




© 








lO 




o> 




o 


CM rj< 


OS 




*} 


i-i CM i-i 00 


00 




o 


OS CO "* 


oo 


i—i 


«o 


H lO CD O) 


t~ 


<M 


o 


io »o co m 


t^. 


i—i 


us 


OOiON 


to 


i—i 


© 


t>- i-l 1-H ^ 


to 




»o 


CM CO 


U5 




© 


T* 


us 














T3 






























^ 










>> 










M 










T3 










d 










a 










8 


: os 






















© 


c3 o 






a 

DQ 


3* 

. — i 03 
03 03 

> b 

















1939] 



Jenkins : Cytogenetic Relationships of Four Species of Crepis 



387 



< 

< a 

3 W 

r} fa 

s i 

I s 

£2 

M <! 
co W 

w w 

11 

«g 

fa o 
O Q 

I* 

fa 

pq 

fa 
o 
W 

H 
O 
fe 

fa 
h3 

fa 

o 
o 



I 



I 


CN 
CN 


o 

CO 


OS 

1-H 


OS 




© 


i—l 




i— 1 


o 


i—l i— 1 


CO 


00 CO CO 

T— 1 


© 

co 


00 ^ 00 


to 


i-H o o o 

rt CO 


© 


©N^H 


■o 


tO N h 
i—i 


* 

© 


b» CN 


OS 

'C 
>> 

GO 

© 
'3 

01 

a 

CO 


si 

■+= 

sfi 

c 

T 

> 

3 


.2 

"c 

1 

s 

K 


P^ 


fe 





388 University of California Publications in Agricultural Sciences [Vol. 6 

none, the F x segregated 59 plants with a stripe to 14 without, and the F 2 
segregated 99 plants with a stripe to 43 without. In the F 2 the deviation 
from a 3 : 1 ratio is not quite significant, since a deviation as large as this 
would be expected in slightly more than one out of ten, from chance 
alone. Progeny were obtained from 5 of the 14 plants in F x that did not 
have a stripe. Four of the 5 segregated F 2 plants with a stripe, indicating 
that the F x plants carried the red-stripe gene though it was not expressed. 
The fifth F x plant did not have a stripe, nor did any of its 10 progeny. 
There is some evidence that the taraxacifolia plant used in the original 
cross was heterozygous, as 2 of its progeny out of a total of 9 were with- 
out a stripe. If this latter progeny is excluded from the total for the F 2 , 
the ratio is 99 : 33, a perfect agreement with a 3 : 1 ratio. 

It is most probable that the F x parents of those F 2 progenies that seg- 
regated red-striped plants although their F x parents had none, did not 
have the proper genetic background for the gene to express itself, but 
that the F 2 recombinations did supply the favorable background ; that 
is, the presence of this character not only requires the presence of the 
gene in the dominant condition, but also requires a definite genie back- 
ground, much as was observed of Harland's (1935) crinkled dwarf in 
Gossypium, which showed different expressions of the character, de- 
pending upon the particular genie milieu in which it had to develop. 
Further evidence for this theory is the wide range of expression of the 
character in F 1? where some plants had such a slight expression that they 
were difficult to distinguish from normal, and an even wider range in F 2 , 
where some of the latter plants were so intense in their expression that 
the color showed through on the upper side of the ligule. 

In crosses between taraxacifolia and Noronhaea, both of which had a 
stripe, all the F x showed the stripe ; and out of 14 F 2 plants 3 had no 
stripe and all these occurred in the progeny of the same F t plant. This 
lack of the red stripe may be due (1) to a slight expression which was 
overlooked in the classification, or (2) to the wrong background for the 
visible expression of the gene. 

In spite of the fact that the numbers are small and the evidence some- 
what conflicting, it appears that the stripe may be referred to a single 
dominant gene which behaves in a normal Mendelian manner ; though 
the expression of the character is dependent, to an appreciable degree, 
upon modifying genes. It is probable, for instance, that if the red-stripe 
gene were introduced into the divaricata background by repeated back- 
crosses, it would segregate in a normal Mendelian fashion but might not 
show the same dominance relationships or expression that it shows in 
taraxacifolia. There is probably enough difference in the genie back- 
ground to suggest that the expression of this gene would be modified in 
the new background. 



1939] Jenkins : Cytogenetic Relationships of Four Species of Crepis 389 

DISCUSSION 

These five entities can be readily distinguished from one another by 
observation ; the morphological differences between them are expressed 
as many small differences in size, color, and shape, affecting almost all 
plant organs. There are a few outstanding qualitative differences be- 
tween them, though these are, taxonomically, of a minor nature ; for 
example, the purple tip on the ligules of canariensis, the ligules wither- 
ing white in andryaloides. 

Two of the species, canariensis on Lanzarote Island of the Canary 
group and Noronhaea on Porto Santo. Island, are geographically well 
isolated from the other two on Madeira. Of the Madeiran species, divari- 
cata and vesicaria andryaloides occupy different ecological stations, the 
former being found only on Promontory San Lorenzo, which is an island 
at high tide, and the latter in the northern highlands of Madeira. In 
spite of the fact that these species have occupied these regions for a long 
time, there is no evidence that they have ever hybridized in nature. 
Vesicaria taraxacifolia, on the other hand, is undoubtedly of more recent 
advent on the island, probably having been introduced by the early 
settlers around Funchal, on the south coast. It is well established there 
and has spread to the north side of the island, particularly around the 
vineyards; furthermore, it is an "aggressive" weedy type and is spread- 
ing. In the north-central part of Madeira, where taraxacifolia and an- 
dryaloides have come into contact, numerous intermediate forms were 
collected and observed by Babcock in 1930. These are undoubtedly nat- 
ural hybrids. But there was no evidence that taraxacifolia had spread 
to the eastern end of the island and hybridized with divaricata. 

The fact that all five entities have the same karyotype and that the 
chromosomes apparently mate up chromomere for chromomere in the 
meiotic prophase of the hybrid, with no subsequent irregularity, would 
indicate that they have essentially the same genie arrangement. It is 
difficult to prove that all five have the same number of genes, though the 
evidence points to this conclusion. There may be minute rearrangements 
and even lack of some particular genes in some of the species ; however, 
if this is so, it is not reflected either in the pairing of the hybrids or in 
a constant elimination of large proportions of gametes. 

The hybridization experiments demonstrate that the species and sub- 
species are able to exchange genes readily. The hybrids are produced 
without difficulty and show a fair measure of fertility ; only a compara- 
tively small proportion of the hybrid recombinations are incapable of 
surviving. The hybrid cultures gave every evidence of being as vigorous 
as the parental species, and were quite as vigorous as the progeny of some 
natural hybrid derivatives of taraxacifolia and andryaloides. 



390 University of California Publications in Agricultural Sciences [Vol. 6 

Al l the evidence is consistent with the view that the five entities have 
a great many gene differences, though the number must remain prob- 
lematical. The most probable assumption is that all the species and 
subspecies possess the same number of genes, but that there are many 
different combinations of alleles. In any one species there must be a 
considerable proportion of heterozygous genes, and since the range of 
variability in the F 2 for most characters is roughly twice that of the 
parents, there must be a higher proportion of heterozygous genes in the 
hybrids between the species and subspecies. 

The prevalent type of F 2 segregation for any single character can be 
satisfactorily explained on the basis of four or five genes with incom- 
plete dominance. But it is not likely that even a probable estimate of the 
total number of genie differences could be obtained by multiplying the 
number of character differences by four or five, as we know that many 
genes, if not all, may influence several characters. It is quite possible 
that a very few genes influencing growth rates at slightly different 
periods of development could produce a large array of character com- 
binations. It would require a very long and extensive breeding program 
to establish with certainty the number of genes influencing any one char- 
acter difference. 

If the total number of basic genes available to these species is desig- 
nated as a, b,c,d,... n, and it is assumed that each gene may have sev- 
eral alleles, which may be designated as a 1 , a 2 , a 3 , . . . & k ; & 1 , & 2 , b 3 , . . . & k , 
etc., the gene population of each species would contain all n genes, but 
many, if not a majority, would be represented by two or more alleles 
clustered around what "Wright (1932) calls an "adaptive peak." Two 
different specific combinations coming together in a hybrid do not 
upset the gene balance, but many of their segregation products (recom- 
binations) are not equally viable : some are so unbalanced as to produce 
lethal or very weak combinations ; in other words, they fall in the "adap- 
tive valleys." 

The genes in one species may be transferred to another, and although 
not all the hybrid combinations are equally successful and many are 
eliminated, it is possible that new and still more harmonious combina- 
tions might be built up. Some offspring might even be adapted to new 
habitats and start an independent line which in time might become eco- 
logically distinct. 

All the evidence indicates that the isolating mechanisms that have been 
built up between these species are due to gene incompatibilities, which 
undoubtedly have arisen by mutation over a long period of time, and 
there is no indication whatever of any chromosomal rearrangements. 
This is rather surprising in view of the ease with which quite radical 
rearrangements were obtained by Navashin and Gerassimova (1936) 



1939] Jenkins : Cytogenetic Relationships of Four Species of Crepis 391 

through the ageing of Crepis seed, which would seem to be a natural 
process. 

In Crepis there are several groups of morphologically closely related 
species with a similar karyotype (Babcock and Cameron, 1934) . It is nat- 
ural to assume that some of the differences in chromosome morphology 
between the groups are due to chromosomal rearrangements. Miintzing 
(1934) found evidence of one inversion between C. divaricata and C. 
dioscoridis, the former from subgenus Barkhausia and the latter from 
subgenus Eucrepis. There have been two additional instances (unpub- 
lished) : C. oporinoides x patula, two distantly related species of the sub- 
genus Eucrepis; and C. canariensis x oporinoides, the former of sub- 
genus Barkhausia and the latter of Eucrepis. In the latter two hybrids 
there were extensive translocations, but there were also very obvious dif- 
ferences in the karyotypes, which would lead one to suspect that there 
had been translocations. 

It is also natural to assume that the species within any one group have 
essentially the same arrangement of genes, particularly in view of the 
fact that the differences in chromosome number and morphology between 
groups are quite striking. Besides the present group, only one other 
closely related group of species with a similar karyotype has been in- 
vestigated. Cave (1936) studied four such species: Crepis foetida, C. 
commutata, C. eritrieensis, and C. Thomsonii, with essentially the same 
result, namely, that there was no evidence of rearrangements. Conse- 
quently, the assumption of a similar arrangement of genes is borne out 
in these two investigations. Whether or not it is true in the whole genus 
will have to be determined by further research. Nevertheless, these two 
instances materially strengthen the evidence for the assumption that 
similar chromosome morphology, of closely related species within this 
genus, indicates structural similarity; accordingly, karyotype studies 
are valuable in determining genetic relationships. 

These species and their close relatives have had a very complex evolu- 
tionary history, involving repeated isolations and hybridizations ; so that 
it is impossible, with the available evidence, to trace their phylogeny in 
any detail. Since there are few qualitative variations differentiating the 
five species and subspecies, and since almost all these variations are pres- 
ent in at least two of them, it is probable that the majority of the specific 
differences were present in the ancestral stock. Nevertheless, some char- 
acter differences have undoubtedly arisen since the separations, for ex- 
ample, the purple tips of the ligules in canariensis, and it is quite likely 
that the quantitative differences have been emphasized in the passage of 
time. The uniformity of the environment on the islands would tend to- 
ward uniformity and less evolution of the species, once they became 
established in a favorable habitat; furthermore, the relatively small 



392 University of California Publications in Agricultural Sciences [Vol. 6 

numbers characteristic of most island species would also automatically 
tend toward still more uniformity (Wright, 1932). 

Since these entities could not be arranged into groups of a higher cate- 
gory, they must have migrated to their present situations at different 
times, or must have come from forms which had already differentiated ; 
either would involve separate migrations. This, with the fact that the 
nearest relatives of canariensis, Noronhaea, divaricata, and andryaloides 
are C. Fontiana, from northwest Africa, and C. Bourgeauii, from south- 
west Spain, would lend support to Cockerel's (1928) hypothesis that the 
indigenous flora of Madeira came from the northeast and, since these are 
oceanic islands, that the facilities for transportation have been available 
at all times. 

It might not be out of place to speculate on the probable future of 
these species, granting that the forces working today continue to operate 
in the same way. Distinct geographic barriers prevent an interchange 
of genes between canariensis and Noronhaea, and their allied species. It 
is reasonable to assume that they will continue to differ progressively 
from each other and from the Madeiran group, and, if they are able to 
survive, will continue to pile up genetic differences which will decrease 
their congruity. 

The situation in Madeira is somewhat different. Andryaloides and 
taraxacifolia are at the present time forming hybrids, backcrosses, and 
complicated segregates. This "hybrid swarm" seems to have many vigor- 
ous and robust plants. It is probable that andryaloides, being a more 
primitive relic, to judge from its perennial habit, restricted range, and 
strict ecological requirement, will in the end be "swamped" by the more 
aggressive taraxacifolia. It is worth noting, however, that this is a very 
slow and gradual process. Since it is highly probable that some of Lowe's 
peculiar forms were hybrid derivatives, the two had come into contact 
over a century ago, perhaps much earlier. Yet most of taraxacifolia 
(in Madeira) and presumably most of andryaloides (in the highlands) 
are still unaffected by the mingling of the two at certain points. But 
taraxacifolia is known to be a montane plant in other countries ; hence, 
in all likelihood, it will gradually invade the highlands, and the mingling 
of the two will continue. 

In any event, andryaloides will have contributed a number of new 
genes to the invader, and this will afford possibilities of segregating out 
new combinations of characters that are even better than the present 
taraxacifolia combinations, making this latter subspecies even more ag- 
gressive. 

Divaricata, with its more restricted range and apparently more primi- 
tive characters (perennial habit, large flowers and leaves) and more 
homozygous expression, which is probably due to the more limited num- 



1939] Jenkins : Cytogenetic Relationships of Four Species of Crepis 393 

bers in the species, will very probably die out because of overgrazing 
by goats, or it may contribute something to the andryaloides-taraxaci- 
folia complex ; so that ultimately there will be one polymorphic species 
with many ecological types. In other words, hybridization may produce 
a degradation as well as a multiplication of forms, and this makes the 
probable phylogenetic history of any species a complex one. 

Those forms which are separated by both geographic and sterility bar- 
riers are, unquestionably, species in the Linnean sense. If the geographic 
barriers are present without the sterility barriers, it is a matter of opin- 
ion what will be the most useful and satisfactory way of treating the 
groups without doing too great violence to the convenient morphogeo- 
graphical system, and at the same time incorporating as much of the 
genetical data as possible. Goodwin (1937) with a very similar situation 
in Solidago feels that on morphological grounds, and for the sake of con- 
venience, the species should be kept distinct even though they do form 
fertile hybrids. 

In a recent paper Clausen, Keck, and Hisey (1936) have proposed a 
scheme based on Turesson's (1929) genecological system. The ecospecies 
(Linnean species) are the smallest units which are kept apart by the aid 
of an inner genetical balance mechanism ; that is, their hybrids are partly 
sterile. The ecotypes (subspecies) produce fertile hybrids and are kept 
apart through their geographical or ecological isolation. In other words, 
the main point is whether the forms have fertile or only partly fertile 
hybrids. The difficulty in this scheme is that, in practice, the fertility 
varies from to 100 per cent, and somewhere along this range a more 
or less arbitrary point must be selected in order to divide the species 
from the subspecies. 

In the present investigation three of these five entities maintain their 
morphological distinctness mainly through geographical and ecological 
isolation, and the other two, andryaloides and taraxacifolia, are becom- 
ing merged. It is clear that the final decision on the taxonomic treatment 
of such genetically close entities or systems must involve some arbitrary 
definitions. Since practical considerations must inevitably go along with 
every scientific approach to these problems, the fact of geographic or 
ecological isolation may properly serve as an adequate basis for the 
recognition of divaricata, Noronhaea, canariensis, and vesicaria. Their 
internal isolating mechanisms are as yet only imperfectly developed; 
that is, they are only on their way to becoming distinct species. However, 
the fact that they are to some degree incongruous and with continued iso- 
lation will probably become more so as time goes on, together with their 
morphological distinctness, would seem to be enough to establish them 
as distinct species. 

Nevertheless, there is still the possibility that, should the territory of 



394 University of California Publications in Agricultural Sciences [Vol. 6 

any one be invaded by another, the two thus coming together would un- 
doubtedly hybridize. It would remain for the botanist of that time to 
determine, through field studies and an investigation of the viability and 
fertility of the hybrid derivatives under natural conditions, the fate of 
the two species involved. 

SUMMARY 

The cytogenetic relationships of four closely related species of Crepis, 
namely, C. canariensis, C. divaricata, C. Noronhaea, and C. vesicaria 
subspp. andryaloides and taraxacifolia, were investigated. The evidence 
presented was derived from (1) a detailed morphological study of the 
parents and the hybrids between them, (2) a comparison of the somatic 
and meiotic chromosome situation of the parents and of the hybrids, and 
(3) the inheritance of a number of characters in the first- and second- 
generation hybrids. 

Among the five entities there were a great many morphological differ- 
ences which affected all parts of the plant. In the hybrids by far the 
greater number of these differences appeared to be the result of the 
presence of a large number of multiple genes. All five had a similar 
karyotype and the chromosome behavior in the hybrids was similar in 
every respect to that in the parents. The internal isolating mechanism 
between them was found to be incomplete, although varying degrees of 
congruity between them were indicated by the comparative fertility of 
the hybrids. For practical taxonomic purposes, the fact of geographic 
or ecologic isolation warrants the recognition of C. divaricata, C. Noron- 
haea, C. canariensis, and C. vesicaria, as species ; whereas andryaloides 
and taraxacifolia must be considered as subspecies of vesicaria, because 
they are hybridizing in nature and are losing their morphological dis- 
tinctness. 



1939] Jenkins : Cytogenetic Relationships of Four Species of Crepis 395 

LITERATURE CITED 

Babcock, E. B., and Cameron, D. R. 

1934. Chromosomes and phylogeny in Crepis. II. The relationships of one hundred 
eight species. Univ. Calif. Publ. Agr. Sci., 6 : 287-324. 

Beadle, G. W. 

1933. Further studies of asynaptic maize. Cytologia, 4:269-287. 

Cave, M. S. 

1936. Cytological and genetical investigations involving Crepis foetida, C. com- 
mutata, C. eritreensis, and C. thomsonii. Unpublished thesis, filed in the Uni- 
versity of California Library. 

Clausen, J., Keck, D. D., and Hiesey, W. M. 

1936. Experimental taxonomy. Carnegie Inst. Wash., Ann. Rept. Div. Plant Biol., 
1935-36:208-214. 



1928. Interspecific hybridization and the origin of species in Nicotiana. Zeitschr. 
f. ind. Abst. u. Vererb., Suppl., 1 : 547-553. 

COCKERELL, T. D. A. 

1928. Aspects of the Madeira flora. Bot. Gaz., 85:66-73. 

Collins, J. L. 

1922. Culture of Crepis for genetic investigations. Jour. Heredity, 13 : 329-336. 

Darlington, C. D. 

1931. The analysis of chromosome pairing in Triticum hybrids. Cytologia, 3 : 21-25. 

DOBZHANSKY, TH. 

1937. Genetics and the Origin of Species (Columbia University Press, New York), 
xvi + 364 pp. 

East, E. M. 

1932. Studies on self -sterility. IX. The behavior of crosses between self -sterile and 
self -fertile plants. Genetics, 17:175-202. 

Goodwin, R. H. 

1937. The cyto-genetics of two species of Solidago and its bearing on their poly- 
morphy in nature. Am. Jour. Bot., 24:425-432. 

Harland, S. C. 

1935. The genetics of cotton. Pt. XIII. A third series of experiments with the 
crinkled dwarf mutant of G. oarbadense L. The cross barbadense crinkled 
x hirsutum crinkled. Jour. Genetics, 31 : 21-26. 

HOLLINGSHEAD, L. 

1930. Cytological investigations of hybrids and hybrid derivatives of Crepis capil- 
laris and Crepis tectorum. Univ. Calif. Publ. Agr. Sci., 6:55-94. 

Kihara, H. 

1929. Conjugation of homologous chromosomes in the genus hybrids Triticum 
x Aegilops and species hybrids of Aegilops. Cytologia, 1 : 1-15. 



396 University of California Publications in Agricultural Sciences [Vol. 6 

Lowe, K. T. 
1868. A Manual Flora of Madeira and the Adjacent Islands of Porto Santo and the 
Desertas (John van Voorst, London), vol. 1, xii + 618 pp. 

McClintock, B. 
1929. A method for making aceto-carmine smears permanent. Stain Tech., 4:53-56. 
1933. The association of non-homologous parts of chromosomes in the mid prophase 
in Zea mays. Zeitschr. f. Zellforsch. u. mik. Anat., 19:191-237. 

Muntzing, A. 

1933. Apomictic and sexual seed formation in Poa. Hereditas, 17 : 131-154. 

1934. Chromosome fragmentation in a Crepis hybrid. Hereditas, 19:284-302. 

Navashin (Nawaschin), M. 

1928. "Amphiplastie" — eine neue karyologische Erscheinung. Zeitschr. f . ind. Abst. 
u. Vererb., Suppl., 2:1148-1152. 

Navashin, M., and Gerassimova, H. 

1936. Natur und Ursachen der Mutationen. III. Ueber die Chromosomenmutationen, 
die in den Zellen von ruhenden Pflanzenkeimen bei deren Altera auf treten. 
Cytologia, 7:437-465. 

Newton, W. C. F., and Pellew, C. 

1929. Primula Jcewensis and its derivatives. Jour. Genetics, 20 -.405-467. 

Smith, F. H. 

1934. The use of picric acid with the gram stain in plant cytology. Stain Tech., 
9:95-96. 

TURESSON, G. 

1929. Zur Natur und Begrenzung der Arteinheiten. Hereditas, 12 : 323-334. 

Wright, S. 

1932. The roles of mutation, inbreeding, crossbreeding and selection in evolution. 
Proc. Sixth Int. Cong, of Genetics, 1 : 356-366. 



EXPLANATION OF PLATE 



PLATE 16 
Rosette Leaves 

1. Crepis canariensis. Note the almost entire margins and the 
winged petioles. 

2. Crepis vesicaria subsp. taraxacifolia x C. canariensis F 4 . Note 
the slenderer petiole and the rounded apex characteristic of 
taraxacifolia. 

3. Crepis divaricata x C. canariensis Y t . Note the dissection on 
the upper half of the leaves very frequently found in divaricata. 

4. Crepis Noronhaea x C. canariensis Fj. Note the lyrate leaves 
and the slender petiole frequently found in Noronhaea. 

5. Crepis vesicaria subsp. andryaloides x C. canariensis F a . Note 
the somewhat modified pinnate dissection characteristic of andry- 
aloides. 

6. Crepis Noronhaea x C. divaricata. Note the characteristic 
divaricata-like dissection as in 3. 

1-6 are approximately %2 their natural size. 

Mature Plants 

7. Crepis canariensis. 

8. Crepis vesicaria subsp. andryaloides. 

9. Crepis divaricata. 

10. Crepis vesicaria subsp. taraxacifolia, spreading form. 

11. Crepis vesicaria subsp. taraxacifolia, erect form. 

12. Crepis Noronhaea. 

The plants 7-12 are growing in 6-inch pots. 



[398] 



UNIV. CALIF. PUBL. AGR. SCI. VOL. 6 



[JENKINS] PLATE 1 6 




I. canaricnsis 






4. F, Noronhaea X 
canariensis 



* 



2. F, taraxacifolia X 
canariensis 




5. F, andryaloides X 
canariensis 




3. F, divaricata X 
canariensis 




6. F, Noronhaea X 
divaricata 




7. canariensis 



8. andryaloides 



9. divaricata 



V " 



N-V 



10. taraxacifolia 



§#> 



1 1. taraxacifolia 



m. 



12. Noronhaea 



[ 399 ]